The market for portable devices such as mobile telephones, personal digital assistants, and MP3 players is highly competitive and it is therefore desirable for manufacturers of such devices to produce distinctive aesthetic effects to attract customers. It is known to produce replaceable coloured plastic cases for mobile telephones, and covers comprising other materials such as leather and fur are also available. Illumination effects have also been used on mobile telephones. It is therefore desirable to produce an effect which would allow a device to stand out from competitors' products, and preferably an effect which is modifiable by a user.